1. Field of the Invention
This invention relates broadly to medical systems. More particularly, this invention relates to perfusion circuits for cardiopulmonary bypass and cardioplegia and methods of using the same.
2. State of the Art
In conventional open-heart surgery, the patient's breast bone is sawed open, the chest is spread apart with a retractor, and the heart is accessed through the large opening created in the patient's chest. The patient is placed on cardiopulmonary bypass and the patient's heart is then arrested using catheters and cannulae which are inserted directly into the large arteries and veins attached to the heart through the large opening in the chest.
Referring to prior art FIG. 1, the prior art bypass perfusion circuit 10 includes an arterial cannula 12 typically passed through the wall of the ascending aorta 14. A venous cannula 16 is passed through the right atrium 18 for withdrawing blood from the patient. The venous cannula 16 is coupled to an approximately eight foot length of ½ inch diameter polyvinyl chloride (PVC) tubing 20 (volume of 304 ml). All prior art systems have mandated the use of at least ½ inch diameter tubing at this location in order to ensure proper blood flow. Tubing 20 leads to a blood reservoir 22 adapted to store a blood volume of 300 to 600 ml. A one foot length of ⅜ inch diameter tubing 24 (volume of 21 ml) couples the reservoir 22 to a centrifugal pump 26 which has a volume of 80 ml. The centrifugal pump 26 is connected to a heart/lung console (not shown) which power the pump. A one foot length of ⅜ inch diameter tubing 28 (volume of 21 ml) transfers blood from the pump 26 to an oxygenator 30 (volume 280 ml). Another one foot length of ⅜ inch diameter tubing 32 (volume of 21 ml) transfers blood from the oxygenator 30 to a forty micron arterial filter 34 (volume of 50 ml). The arterial filter 34 is adapted to capture gaseous and fatty embolisms. From the filter 34, an eight foot length of ⅜ inch diameter tubing 36 (volume 168 ml) completes the circuit back to the arterial cannula 12. The reservoir 22, oxygenator 30, and arterial filter 34 are an integrated unit 38. Nevertheless, tubings 24, 28, 32 are required to connect the various sections thereof. Blood is pulled from the patient through the venous cannula 16, circulated through the tubing, reservoir, pump, oxygenator and filter, which are together referred to as the perfusion circuit 10, and back to the patient through the arterial cannula 12. The entire bypass circuit 10 is mounted on a pole fixed to the console (now shown) which powers the centrifugal pump 26, and is thus constrained to the location of the console.
Prior to use, the two lengths of eight foot tubing 20, 36 of the circuit 10 are coupled together at a pre-bypass filter (not shown) having an 80 ml volume. One length of the tubing is then decoupled from about the pre-bypass filter and the circuit is primed with an isotonic solution, e.g., saline, to remove air and any other impurities from within the components and tubing. The priming volume is relatively high, calculated from the above stated individual volumes of the tubing and components to be approximately 1325 to 1625 ml (not including the cannulae). Note that the ½ inch diameter tubing has a volume of 38 ml/foot, and ⅜ inch diameter tubing has a volume of 21 ml/foot.
After the perfusion circuit 10 is primed with saline, the pre-bypass filter is removed and respective ends of the circuit are coupled to the arterial and venous cannulae 12, 16.
Referring to prior art FIG. 2, a cardioplegia circuit 40 is then coupled to the heart. The cardioplegia circuit 40 generally includes a roller pump 42 which pulls blood from the oxygenator and feeds the blood into the heart. To the circuit 40, cardioplegia fluid 44 is added. The cardioplegia fluid 44 is generally potassium suspended in a one liter isotonic solution. A length of flexible tubing 46 extends from the roller pump 42 to a bubble trap 48 and a catheter 50 extending from the bubble trap 48 into the heart 15. The components and tubing of the cardioplegia circuit must also be primed with approximately 300 to 400 ml of an isotonic solution to remove air and foreign matter prior to use. After priming, the roller pump 42 is operated to induce cardioplegia.
Once the perfusion circuit pump is operated, cardioplegia is induced and the patient's blood is oxygenated outside the body and circulated back to the patient. When the perfusion pump is operated, the priming saline is also circulated through the patient's body.
This standard procedure is undesirable for several reasons. First, the relatively large priming volume, and particularly the length of tubing, of the system requires that the patient's blood come into contact with a large non-vascular surface area for a relatively long period of time. When blood contacts plasticizer components such as the tubing, there tends to be an inflammatory response by the body. This is so even when the tubing and other components are coated with modern anti-inflammatory coatings. This response can compromise the recovery of the patient.
Second, there are instances in prior art perfusion circuits where a section of the tubing or cannulae kinks, inhibiting blood flow through the perfusion circuit. In such a situation, the pump may draw in air through the reservoir, which is open to the atmosphere, and circulate the air into the patient's vascular system. This is extremely dangerous to the patient and may even be deadly.
Third, the large amount of isotonic fluid required for priming the perfusion bypass and cardioplegia circuits is circulated into the patient's body in addition to units of blood that may have been administered to the patient prior to the procedure. This extraordinary volume of fluid in the human body taxes the patient, as the kidneys are forced to process a substantial amount of additional fluid.